Liquid level sensor

ABSTRACT

A liquid level detecting apparatus resistant to the deterioration and corrosion thereof is provided while holding down an increase in the manufacturing cost thereof. This apparatus  1  has fixed electrodes  3  on which a sliding contact point  5  is slidingly moved in accordance with a fluctuation of a float, and a first resistor  4  connected to the fixed electrodes  3,  the fixed electrodes  3  being made of at least silver (Ag), palladium (Pd) and a glass component, a weight ratio of the silver (Ag) to palladium (Pd) being not lower than 75 wt. % to not higher than 25 wt. %, a sliding portion  7  of the fixed electrodes  3  on which the sliding contact point  5  is slidingly moved being covered with second resistors  8  a resistance value of which is lower than that of the first resistor  4.

TECHNICAL FIELD

[0001] This invention relates to a liquid level detecting apparatus, andmore particularly to a liquid level detecting apparatus provided withfixed electrodes on which a sliding contact point is slidingly moved inaccordance with a fluctuation of a float.

BACKGROUND ART

[0002] The related art liquid level detecting apparatuses include, forexample, the apparatus disclosed in JP 59-93102 U.

[0003] In such a liquid level detecting apparatus, the materials for thefixed electrodes generally include a mixture of silver-palladium powder(AgPd) and glass. It is known that the fixed electrodes are obtained byprinting on an insulating circuit board paste of a mixture of silver(Ag) powder, palladium (Pd) powder, glass powder and a solvent, dryingthe printed paste, and thereafter calcining the resultant product. Thesilver (Ag) has a low electric resistance, and is excellent in theconductivity but the us of the silver in the fuel causes the silver tobe deteriorated by or corroded with, for example, sulfur, water andalcohol contained in the fuel. This causes an imperfect conduction ofthe silver (Ag). Therefore, the deterioration and corrosion preventingmeasures have been taken by adding an anti-deterioration andanti-corrosion substance including palladium (Pd) to the silver (Ag).

[0004] However, since the fixed electrodes use palladium (Pd), a noblemetal, the material cost increases, and a liquid level detectingapparatus becomes expensive. Under the circumstances, one of the objectsof the present invention is to provide a liquid level detectingapparatus resistant to the deterioration and corrosion of the fixedelectrodes while holding down a cost increase.

DISCLOSURE OF INVENTION

[0005] To achieve this object, the present invention provides a liquidlevel detecting apparatus having fixed electrodes on which a slidingcontact point is slidingly moved in accordance with a fluctuation of afloat, and a first resistor connected to the fixed electrodes, the fixedelectrodes being made of at least silver (Ag), palladium (Pd) and aglass component, a weight ratio of the silver (Ag) to palladium (Pd)being set not lower than 75 wt. % and not higher than 25 wt. %, slidingportions of the fixed electrodes on which the sliding contact point isslidingly moved being covered with second resistors the resistance valueper unit area of which is lower than that of the first resistors. Thestructure thus formed enables a liquid level detecting apparatusresistant to the deterioration and corrosion of the fixed electrodes tobe provided while holding down a cost increase.

[0006] The second resistors contain at least ruthenium oxide (RuO₂), andhave a sheet resistivity of not higher than 1.5 Ω/mm². Owing to thisstructure, a liquid level detecting apparatus resistant to thedeterioration and corrosion of the fixed electrodes can be providedwhile holding down a cost increase.

[0007] The thickness of a film of each of the fixed electrodes is notlarger than 12 μm. Owing to this structure, a liquid level detectingapparatus resistant to the deterioration and corrosion of the fixedelectrodes can be provided while holding down a cost increase. Moreover,the quantity of the material in use for forming the fixed electrodes 3can be reduced, and, in addition, the quantity of the expensivepalladium (Pd) in use can also be reduced. This enables a liquid leveldetecting apparatus capable of further reducing the cost to be provided.

[0008] Each of the fixed electrodes has a weight ratio of a combinationof the silver (Ag) and palladium (Pd) to the glass component set so thatthe weight percentage of the glass component is not higher than 15 wt.%. Owing to this structure, a liquid level detecting apparatus having ahigh resistance to the deterioration and corrosion of the fixedelectrode can be provided while holding down a cost increase. When thecontent of the glass component is held down to a low level, excellentsoldering characteristics of the fixed electrodes can be obtained.

[0009] The present invention relates to a liquid level detectingapparatus having fixed electrodes on which a sliding contact isslidingly moved in accordance with a fluctuation of a float, and a firstresistor connected to the fixed electrodes, each of the fixed electrodesbeing made of at least silver (Ag), palladium (Pd) and a glasscomponent, a weight ratio of the silver (Ag) to palladium (Pd) being setnot lower than 80 wt. % and not higher than 20 wt. %, a sliding portionof the fixed electrode on which the sliding contact point is slidinglymoved being covered with second resistors the resistance value per unitarea of which is lower than that per unit area of the first resistor.Owing to this structure, a liquid level detecting apparatus resistant tothe deterioration and corrosion of the fixed electrodes can be providedwhile holding down a cost increase.

[0010] The second resistors contain at least ruthenium oxide (RuO₂), andhave a sheet resistivity of not higher than 1.0 Ω/mm². Owing to thisstructure-a liquid level-detecting apparatus having a resistance to thedeterioration and corrosion of the fixed electrodes can be providedwhile holding down a cost increase.

[0011] The thickness of a thin film of each of the fixed electrode isnot larger than 12 μm, or not smaller than 7.5 μm. Owing to thisstructure, a liquid level detecting apparatus resistant to thedeterioration and corrosion of the fixed electrodes can be providedwhile holding down a cost increase. Since it is possible to reduce thequantity of the material in use for forming the fixed electrodes andthat of the expensive palladium (Pd), a liquid level detecting apparatuscapable of further reducing the cost can be provided.

[0012] Each of the fixed electrodes has a weight ratio of a combinationof the silver (Ag) and palladium (Pd) to the glass component set so thatthe weight percentage of the glass component is not higher than 15 wt.%, or each of the fixed electrodes has a weight ratio of a combinationof the silver (Ag) and palladium (Pd) to the glass component set so thatthe weight percentage of the glass component is not lower than 10 wt. %.Owing to this structure, a liquid level detecting apparatus resistant tothe deterioration and corrosion of the fixed electrodes can be providedwhile holding down a cost increase. Moreover, since the content of theglass component is held down to a low level, it becomes possible toobtain excellent soldering characteristics of the fixed electrodes.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 is a front view of a first mode of embodiment of thepresent invention;

[0014]FIG. 2 is a sectional view taken along the line A-A in FIG. 1;

[0015]FIG. 3 is a front view of a second mode of embodiment of thepresent invention; and

[0016]FIG. 4 is a sectional view taken along the line B-B in FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] A first mode of embodiment of the present invention will bedescribed with reference to FIGS. 1 and 2. This mode of embodiment willbe described on the basis of a case where the present invention isapplied to a liquid level detecting apparatus for a vehicle, such as anautomobile.

[0018] This liquid level detecting apparatus 1 include an insulatingcircuit board 2 of a ceramic material, a plurality of fixed electrodes 3provided on this circuit board 2, a first resistor 4 provided astridethe plural fixed electrodes 3, a sliding member 6 provided with asliding contact point 5 slidingly moved on the fixed electrodes 3,second resistors 8 provided on a sliding portion 7 of the fixedelectrodes on which the sliding contact point 5 is slidingly moved, andregulating resistors 9 adapted to regulate a resistance value of thefirst resistor 4.

[0019] Each of the fixed electrodes 3 is made of at least silver (Ag),palladium (Pd) and a glass component. The glass component is made ofboro-silicate lead glass and bismuth oxide. The fixed electrodes 3 areformed by turning a mixture of the materials thereof including theabove-mentioned silver (Ag), palladium (Pd) and a glass component into apaste with a solvent. This paste type electrode material is formed onthe circuit board 2 by means of screen printing techniques, and fixedelectrodes 3 are thereafter formed via drying and calcining steps.

[0020] The fixed electrodes 3 are provided with the sliding portion 7, asolder-connected portion 10 and measuring lands 11. The fixed electrodes3 constituting the sliding portion 7 are formed plurally. Thesolder-connected portion 10 is formed solely. This solder-connectedportion 10 is formed to a substantially square shape in this mode ofembodiment. A conductive member, such as a lead wire (not shown) isconnected to the solder-connected portion 10 by soldering (not shown).The liquid level detecting apparatus 1 outputs a detection signal to ameter (not shown) through the conductive member.

[0021] The measuring lands 11 are members which a detecting needle (notshown) of a measuring unit contacts, the measuring unit being adapted tomeasure the resistance values of the portions of the first resistor 4which are between the measuring lands 11. When a resistance value of thefirst resistor 4 is not a predetermined resistance value, the regulatingresistors 9 are trimmed by a laser trimming method and the like so as toprovide removal portions 12, and the resistance value of the firstresistor 4 is thereby regulated.

[0022] The first resistor 4 is made of a material containing at leastruthenium oxide (RuO₂). The second resistors 8 and regulating resistors9 are also made of the same material as the first resistor 4. However,the resistance values of these resistors 4, 8, 9 are set so that theresistance values are different from each other. Especially, theresistance value of the second resistors 8 is set so that the resistancevalue is lower than that of the first resistor 4.

[0023] The first resistor 4 is calcined on the plural fixed resistors 3,which constitute the sliding portion 7, in such a manner that the firstresistor 4 strides over the fixed electrodes 3. The first resistor 4 isalso connected to the solder-connected portion 10. The first resistor 4is formed by means of screen printing techniques and the like in thesame manner as the fixed electrodes 3, and then subjecting the resultantproduct to the drying and calcining steps is formed. The secondresistors 8 and regulating resistors 9 are also formed through the samesteps as the first resistor 4.

[0024] The sliding contact point 5 is fixed to the sliding member 6 bycalking. Suitable materials for the sliding contact point 5 include acopper (Cu)-nickel (Ni)-zinc (Zn) alloy (German silver or nickel silver)and a nickel (Ni)-chromium (Cr) alloy. The sliding member 6 is suitablymade of a copper (Cu)-nickel (Ni)-Zinc (Zn) alloy because this alloy iseasily subjected to a processing step, such as a calking step, resistantto sulfur, and, in addition, inexpensive.

[0025] The sliding contact point 5 is a member adapted to be slidinglymoved on the sliding portion 7 of the fixed electrodes 3 in accordancewith the fluctuation of a float (not shown) floating on a level of thefuel. On the sliding portion 7 of the fixed electrodes 3, secondresistors 8 are formed. The second resistors 8 are formed so as to coverthe sliding portion 7.

[0026] The second resistors 8 are made of a material containing at leastruthenium oxide (RuO₂). Therefore, the second resistors 8 have a highabrasion resistance, and can prevent the sliding portion 7 from beingworn. The sliding portion 7 is covered with the second resistors 8having a high abrasion resistance and a high sulfurization resistance.This can prevent at the portion of the second resistors 8 which isengaged with the sliding contact point 5 the silver (Ag) contained inthe slilding portion 7 (fixed electrodes 3) from being sulfurized by thesulfur content of gasoline, the fuel for an automobile, and from therebygenerating silver sulfide.

[0027] In this mode of embodiment, the sheet resistivity of the secondresistors 8 is set not higher than 1.5 Ω/mm², and preferably to 1.0Ω/mm². Since the second resistors 8 have such a sheet resistivity value,it is possible that the second resistors 8 as resistors containingruthenium oxide (RuO₂) rarely give an electric influence with a lowresistance upon a portion between the sliding contact point 5 andsliding portion 7 while obtaining an abrasion resistance andsulfurization resisting characteristics.

[0028] In the fixed electrodes 3 forming the sliding portion 7,solder-connected portion 10 and measuring lands 11 which are made of thesame material and the same components, the weight ratio of a combinationof the silver (Ag) and palladium (Pd) to glass component is 85:15, i.e.17:3 in this mode of embodiment. Owing to this structure, the abrasionresistance of the sliding portion 7 is obtained at the second resistors8. Since the fixed electrodes contain a large quantity of conductivematerials, such as silver (Ag) and palladium (Pd) with the ratio of theglass component held down, the fixed electrodes have an excellentelectric performance, and the connection of solder in the solderconnected portion 10 can also serve to obtain satisfactory performancethereof. Since the content of the glass component of the fixedelectrodes 3 is held down to a low level, the quantity of the glasscomponent migrating from the fixed electrodes 3 to the second resistors8 when the second resistors 8 are calcined can be held down to a lowlevel, and it becomes possible to restrain the electric noise occurringwhen the sliding contact point 5 and second resistors 8 contact eachother. The ratio of the conductive materials to the glass component isnot limited to the above-mentioned level. When the quantity of theconductive materials is not lower than 85 wt. % with that of the glasscomponent not higher than 15 wt. %, fixed electrodes having both asatisfactory electric performance and a satisfactory solder connectingoperation can be obtained.

[0029] In this mode of embodiment, the weight ratio of silver (Ag) topalladium (Pd) in the fixed electrodes 3 is 75:25, i.e. 3:1. When thequantity of palladium (Pd) in use is thus reduced, the material cost canbe reduced, and, in its return, the cost of manufacturing the liquidlevel detecting apparatus 1 can also be reduced. The ratio of the silver(Ag) to palladium (Pd) is not limited to the above-mentioned level. Aslong as the content of the silver is not lower than 75% with that of thepalladium not higher than 25 wt. %, a satisfactory electric performanceis obtained, and the manufacturing of the apparatus can be done at asatisfactory cost.

[0030] The thickness of the films of the fixed electrodes 3 is notlarger than 12 μm. Owing to this structure, the quantity of thematerials in use for forming the fixed electrodes 3 can be reduced, and,in its return, the quantity of palladium (Pd), a noble metal in use canbe reduced. This enables the manufacturing cost to be further reduced.

[0031] A second mode of embodiment of the present invention will now bedescribed with reference to FIG. 3 and FIG. 4. The parts of thisembodiment identical with and equivalent to those of the first mode ofembodiment will be designated by the same reference numerals, anddetailed descriptions thereof will be omitted.

[0032] The fixed electrodes 103 in the second mode of embodiment arealso made of, at least, silver (Ag), palladium (Pd) and a glasscomponent. The fixed electrodes 103 form a sliding portion 107, a solderconnected portion 100 and measuring lands 111.

[0033] Second resistors 108 are made of at least a material containingruthenium oxide (RuO₂). A first resistor 4 and regulating resistors 9are also made of the same material as the second resistors 108. However,the resistance values of these resistors 4, 108, 9 are set so that theresistance values are different from each other, and, especially, theresistance value of the second resistors 108 is set so that theresistance value becomes lower than that of the first resistor 4. Thesecond resistors 108 are formed so as to cover the sliding portion 107of the fixed electrodes 103 therewith.

[0034] The sliding portion 107 is thus covered with the second resistors108 with a high abrasion resistance and a high sulfurization resistance.This can prevent at the portion of the second resistors 108 which isengaged with the sliding contact points 105 described later the silver(Ag) contained in the sliding portion 107 (fixed electrodes 103) frombeing sulfurized by the sulfur content of gasoline, the fuel for anautomobile, and from thereby generating silver sulfide.

[0035] The sliding contact points 105 are made integral with a slidingmember 106. The sliding contact points 105 are formed by the pressingwork. Suitable materials for the sliding contact points 105 and slidingmember 106 include a copper (Cu)-nickel (Ni)-zinc (Zn) alloy (Germansilver or nickel silver). This material is suitable since the materialis easily processed, resistant to the sulfurization thereof, and, inaddition, inexpensive.

[0036] The sliding contact points 105 are bent so that the slidingcontact points engage a circuit board 2 in the substantiallyperpendicular direction, and the bent sliding contact points 105 have asuitable length. Therefore, it becomes possible to, for example,increase a sliding frequency lost due to the wear on the sliding contactpoints, and this causes in its return the lifetime of the slidingcontact points 105 to be prolonged.

[0037] Although the sliding contact points 105 in this mode ofembodiment are made integral with the sliding member 106, the slidingcontact points and sliding member may be formed separately as in thefirst mode of embodiment.

[0038] In the second embodiment, the sheet resistivity of the secondresistor 108 is set not higher than 1.0 Ω/mm². Owing to such aresistance value, it is possible that the second resistors as resistorscontaining ruthenium oxide (RuO₂) rarely give an electric influence witha low resistance upon a portion between the sliding contact point 105and sliding portion 107 while obtaining an abrasion resistance andsulfurization resisting characteristics.

[0039] In the fixed electrodes 103 in this mode of embodiment, theweight ratio of a combination of silver (Ag) and palladium (Pd) to aglass component is set to 90:10, i.e. 9:1. Owing to this structure, theabrasion resistance of the sliding portion 107 is obtained at the secondresistors 108. Since the fixed electrodes contain large quantities ofconductive materials, such as silver (Ag) and palladium (Pd) with theratio of the glass component held down, the fixed electrodes have anexcellent electric performance, and the connection of solder in thesolder-connected portion 110 can also serve to obtain a satisfactoryperformance thereof.

[0040] The content of the glass component in the fixed electrodes 103 isheld down to a low level. Therefore, when the second resistors 108 arecalcined, the glass component in the fixed electrodes 103 can hold downthe quantity of the glass component which migrates therefrom to the sideof the second resistors 108, and this enables electric noise occurringwhen the sliding contact point 105 and second resistor 108 contact eachother to be held down.

[0041] The ratio of the content of the conductive materials to that ofthe glass component is not limited to the above-mentioned level. Whenthe content of the glass component is in the range from not lower than10 wt. % to not higher than 15 wt. % with that of the conductivematerials in the range from not higher than 90 wt. % to not lower than85 wt. %, a satisfactory electric performance is obtained, and theconnection of solder can also be executed satisfactorily.

[0042] The weight ratio of the silver (Ag) to palladium (Pd) containedin the fixed electrodes 103 in this mode of embodiment is set to 80:20,i.e. 4:1. When the quantity of palladium (Pd) in use is thus reduced,the reduction of the cost can be effected, and, in its return, the costof manufacturing the liquid level detecting apparatus can also beattained. The ratio of the silver (Ag) to palladium (Pd) is not limitedto the above-mentioned level. As long as the content of the silver isnot lower than 80 wt. % with that of the palladium not higher than 20wt. %, a satisfactory electric performance is obtained, and themanufacturing of the apparatus can be done at a satisfactory cost.

[0043] The thickness of the films of the fixed electrodes 103 is notlarger than 12 μm, and preferably not smaller than 7.5 μm. Owing to thisstructure, the quantity of the materials used to form the fixedelectrodes 103 can be reduced, and, in its return, the quantity of thepalladium (Pd), a noble metal in use can also be reduced. This enablesthe manufacturing cost to be further reduced.

Industrial Applicability

[0044] The present invention can be utilized for a liquid leveldetecting apparatus resistant to the deterioration and corrosionthereof, with an increase in the manufacturing cost held down.

1. A liquid level detecting apparatus provided with fixed electrodes onwhich a sliding contact point is slidingly moved in accordance with afluctuation of a float, and a first resistor connected to the fixedelectrodes, characterized in that the fixed electrodes are made of atleast silver (Ag), palladium (Pd) and a glass component, a weight ratioof the silver (Ag) to the palladium (Pd) being set not lower than 75 wt.% to not higher than 25 wt. %, a sliding portion of the fixed electrodeson which the sliding contact point is slidingly moved being covered withsecond resistors a resistance value per unit area of which is lower thanthat per unit area of the first resistor.
 2. A liquid level detectingapparatus according to claim 1, wherein the second resistors contain atleast ruthenium oxide (RuO₂), and have a sheet resistivity of not higherthan 1.5 Ω/mm².
 3. A liquid level detecting apparatus provided withfixed electrodes on which a sliding contact point is slidingly moved inaccordance with a fluctuation of a float, and a first resistor connectedto the fixed electrodes, characterized in that the fixed electrodes aremade of at least silver (Ag), palladium (Pd) and a glass component, aweight ratio of the silver (Ag) to the palladium (Pd) being set to notlow r than 80 wt. % to not higher than 20 wt. %, a sliding portion ofthe fixed electrodes on which the sliding contact point is slidinglymoved being covered with second resistors a resistance value per unitarea of which is lower than that per unit area of the first resistor. 4.A liquid level detecting apparatus according to claim 1 or 3, whereinthe second resistors contain at least ruthenium oxide (RuO₂), and have asheet resistivity of not higher than 1.0 Ω/mm².
 5. A liquid leveldetecting apparatus according to claim 1 or 3, wherein the thickness offilms of the fixed electrodes is not larger than 12 μm.
 6. A liquidlevel detecting apparatus according to claim 3, wherein the thickness ofthe films of the fixed electrodes is not smaller than 7.5 μm.
 7. Aliquid level detecting apparatus according to claim 1 or 3, wherein,concerning a weight ratio of a combination of the silver (Ag) andpalladium (Pd) to the glass component, the content of the glasscomponent is not higher than 15 wt. %.
 8. A liquid level detectingapparatus according to claim 3, wherein, concerning a weight ratio of acombination of the silver (Ag) and palladium (Pd) to the glasscomponent, the content of the glass component is not lower than 10 wt.%.